1. Field of Invention
The present invention relates generally to the implantation of a cardiac pacing device used for cardiac resynchronization therapy (CRT). More specifically, the present invention relates to a realtime method for CRT candidate screening, for optimizing the placement of one or more leads, and for determining optimal settings for cardiac pacing devices.
2. Description of Related Art
Heart failure afflicts about twenty-five million people worldwide, with about two million new cases diagnosed each year. In the United States, hospitalization for heart failure amounts to more than 6.8 million days a year, and the total cost of treatment is more than $38 billion annually, which is increasing as the population ages. The prevalence of congestive cardiac failure is also increasing due to improved survival from both myocardial infarction and hypertension that has resulted from the use of drug therapies, such as angiotension-coverting enzyme inhibitors, beta-blockers, and digoxin. Nevertheless, many patients remain markedly symptomatic despite maximal medical therapy. Furthermore, patients with left ventricular failure are at an increased risk of progressive heart failure or sudden death.
In some heart patients, congestive cardiac failure affects the synchronous beating of the ventricles. Accordingly, the left ventricle is not able to pump blood efficiently to supply the body with needed oxygen and nutrients. In approximately 30% of patients with heart failure, an abnormality in the heart's electrical conduction system, called an intraventricular conduction delay or bundle branch block, causes the left ventricle to beat in an asynchronous fashion. This greatly reduces the efficiency of the left ventricle in patients whose hearts are already damaged. In addition, the right and left ventricles begin to beat slightly out of phase instead of beating simultaneously.
A significant minority of patients with congestive heart failure have marked prolongation of the QRS complex of their electrocardiographic (ECG) profile, which represents the time it takes for the depolarization of the ventricles. The prolongation is an indicator of intraventricular conduction abnormality and is associated with decreased left ventricular systolic function. The development of new QRS prolongation is associated with reduced left ventricular function.
Normally, electrical activation is conducted by the His bundle and Purkinje system, and an impulse spreads transmurally from the septum to multiple paraseptal regions resulting in synchronous contraction of the ventricles. Many patients with heart failure have poor electrical conduction in the heart that results in a pattern called left bundle branch block (LBBB) or intraventricular conduction delay. In these patients, the duration of the QRS complex may exceed 130 milliseconds (ms) compared with a normal duration of less than 100 ms. In LBBB, the left ventricle is activated belatedly throughout the septum from the right ventricle, with anteroseptal crossing preceding inferioseptal crossing. The latest activation is in the posterior inferior aspect of the left ventricle, often remote from the base.
Additionally in patients with LBBB, the delay between the onset of left and right ventricle systole may be prolonged to 85 ms resulting in significantly later aortic opening, aortic valve closure, and mitral valve opening. LBBB does not affect the timing of right ventricle events, and the delay in the left ventricle events leads to a reversal of the usual sequence of right and left ventricle systole. In addition, the range of isovolumic contraction times in patients with LBBB is wide (20–100 ms), suggesting heterogeneity of left ventricle activation. The delay in aortic valve closure leads to a relative decrease in the duration of left ventricle diastole. In patients with LBBB, prolonged depolarization or abnormal depolarization may result in regional myocardial contraction into early diastole, causing a delay of mitral valve opening with prolongation of left ventricle isovolumic relaxation time of up to 300% and shortened left ventricle filling time. LBBB is also associated with abnormal diastolic function on Doppler echocardiography examination. Further, left ventricle intraventricular conduction delay may add significantly to dyssynchrony, particularly in ischemic heart disease.
In patients with an intraventricular conduction defect or with LBBB, cardiac resynchronization therapy (CRT) shortens the duration of the QRS complex and has been shown to improve the patient's symptoms markedly. CRT is the use of a specialized pacemaker to improve contraction coordination of the left ventricle. The specialized pacemaker may also be programmed to coordinate the beating of the two ventricles by pacing the left ventricle individually to match the beating of the right ventricle or both ventricles simultaneously. It has been shown that resynchronization of abnormal intraventricular and interventricular asynchrony with left ventricular or biventricular pacing may symptomatically improve patients with severe ventricular failure. While the results have been positive, most studies have shown that approximately 30% of patients do not obtain any measurable benefit from the therapy. It is now being tested to see if this therapy will increase the duration of life.
In biventricular pacing, one wire or catheter is implanted in the right ventricle and another is threaded into a vein, the coronary sinus, which drains into the right atrium to pace the left ventricle. The coronary sinus catheter is then guided to the lateral or posterior part of the left ventricle. Alternatively, a left ventricular lead can be implanted by thoracotomy (i.e., through a small incision between the ribs, the lead is implanted on the surface of the left ventricle) or even by crossing the atrial septum and inserting the lead inside the left ventricle. Yet, the exact and best position for each catheter position is difficult to determine at the time of insertion. In fact, there are no physiological means to determine the best site at the time of lead placement except possibly the use of echocardiography, which is time consuming and poses a problem in keeping the operative field sterile.
In addition, it is difficult to predict the effectiveness of CRT before the insertion of the cardiac pacing device. Currently, physicians often measure a decrease in QRS duration after biventricular pacing to evaluate CRT. However, the decrease in the QRS duration does not correlate well with the improvement of cardiac function in some patients. Other parameters have been also used to determine the effectiveness of CRT, such as improvement of New York Heart Association (NYHA) classification score, six-minute hall walk results, etc. However, these parameters cannot be evaluated in real time and do not provide information that physicians need to know at the time of lead and device implantation to determine if the patient will benefit from CRT. Finally, it is not clear if one lead implanted into the coronary sinus is as good as two leads implanted into the right and left ventricles.
Another problem encountered, particularly with the use of dual-chamber pacemakers, is the proper setting of the so-called “A-V delay interval.” Basically, the A-V delay interval refers to the time interval between a ventricular stimulation pulse and a preceding atrial depolarization. Because the sequence of atrial and ventricular pacing is vital to the efficiency of the heart as a pump, a non-optimal A-V delay interval can seriously impact heart performance. Indeed, relatively small departures from the optimal A-V delay interval value can greatly reduce the hemodynamic contribution of the atria in patients with congestive heart failure.
At present, physicians select and program the A-V delay interval empirically. Since the hemodynamic contribution of the atrial depolarization to cardiac output is well known, every effort is made to select the optimal A-V delay interval for a given patient. However, the optimal A-V delay value can vary over time as the patient ages or the disease state changes.
Therefore, there is a need for a way to provide a reliable prediction for whether a patient would be a good candidate for cardiac resynchronization therapy, for a way to determine the optimal placement of leads of a cardiac pacing device in realtime while the pacing device is implanted into the patient, and for a way to optimize the selection of the A-V delay interval, both during the initial placement of a dual-chamber pacemaker and during follow-up evaluations.